1. Field of the Invention
This invention relates to a pneumatic tire comprising sipes in a tread portion and a vulcanization mold for use in the production of such a pneumatic tire.
2. Description of Related Art
Heretofore, there has been known a pneumatic tire having a tread pattern as shown in FIGS. 6 and 7. A plurality of land portions, blocks 14 in the illustrated embodiment are defined in a tread portion 11 by a plurality of wide-width grooves, such as main grooves 12 extending in a circumferential direction of the tire and lateral grooves 13 crossing with the main grooves 12. A plurality of narrow-width sipes 15 extend substantially in an axial direction of the tire and are formed in each of the blocks 14. In this case, all of these main grooves 12, lateral grooves 13 and sipes 15 extend in a radial direction of the tire, so that even if the tread portion 11 is worn in the running of the tire, the plane shape of each of the main groove, lateral groove 13, block 14 and sipe 15 is substantially unchangeable.
In the conventional pneumatic tire, however, there are problems that as the wearing of the tread portion 11 progresses, the groove depth of each of the main groove 12, lateral groove 13 and sipe 15 is gradually made shallow to degrade the drainage property and also the height of the block 14 is lowered and the rigidity of the block in front and rear directions and lateral direction 3 is gradually increased to degrade the steering stability on wet road, the braking property on wet road and the ride comfort against vibrations on dry road.